Unless otherwise indicated herein, the materials described in this section are not prior art to the claims and are not admitted to be prior art by inclusion in this section.
In a cellular wireless communication system, the downlink air interface from each base station typically has limited capacity to carry signaling information, such as general system information and user-specific information such as paging messages and the like. For example, in a system where the air interface is time division multiplexed, only certain periodically recurring time segments may be available to carry signaling information. Further, in a system where the air interface is frequency division multiplexed, only certain sub-carriers may be available to carry signaling information, perhaps also on a periodically recurring time basis.
In such systems, an issue may be how to allocate the limited air interface signaling capacity for use to carry user-specific signaling information.
The Long Term Evolution (LTE) air interface is a good example of such a scenario. LTE employs orthogonal frequency division multiplexing (OFDM), where symbols are distributed on subcarriers across a defined bandwidth, such as 10 MHz for instance. Periodically over time, such as every 0.5 ms for example, each LTE base station (“eNodeB”) may transmit control signaling that occupies defined resource elements (OFDM symbols) in up to three 67 μs time segments across the bandwidth.
Certain resource elements at each 0.5 ms signaling period may be reserved for use to carry reference symbols that essentially define a pilot signal for user equipment (UE) to monitor coverage. Still other resource elements may be reserved to define a physical control format indicator channel (PCFICH), which carries signaling overhead information such as an indication of how many 67 μs time segments are being used for control signaling. Remaining resource elements may then be used to carry one or more physical downlink control channels (PDCCH), each carrying downlink control information (DCI) including air interface resource assignments and other control information for one or more particular UEs.
In LTE as currently defined by way of example, each PDCCH provides UE-specific control information within a number of control channel elements (CCE), each of which is provided as nine resource element groups (REG), with each REG being four resource elements, mapping four quadrature phase shift keying (QPSK) symbols, for a total of 36 QPSK symbols per CCE. The CCEs are numbered with identifiers, and the base station may allocate particular CCEs to particular UEs by specifying the allocations in the PCFICH, with reference to CCE IDs and UE IDs.